1. If 'mat-form-field' is an Angular component, then verify that it is part of this module. 2. If 'mat-form-field' is a Web Component then add 'CUSTOM_ELEMENTS_SCHEMA' to the '@NgModule.schemas' of this component to suppress this message. code example

Example 1: 'mat-form-field' is not a known element

@NgModule({
  imports: [
  	...
	MatFormFieldModule,
    MatInputModule,
    ...
  ]
})

Example 2: 'mat-label' is not a known element:

// common-widgets.module.ts
import { MatFormFieldModule } from '@angular/material/form-field';
import { MatInputModule } from '@angular/material/input';

@NgModule({
  imports: [
    MatFormFieldModule,
    MatInputModule
  ]
})
export class CommonWidgetsModule { }

Example 3: 'mat-form-field' is not a known element:

@NgModule({
  imports: [
    MatButtonModule,
    MatFormFieldModule,
    MatInputModule,
    MatRippleModule,
 ]
 ....